The greater part of a lead storage battery is used for starting an automobile. When a charging condition of the lead storage battery is not sufficient, the battery is short of its residual capacity so that the automobile becomes unable to be started. If the charging condition is known previously, countermeasures such as execution of supplementary charging etc. may be taken before stopping the automobile. However, it has been impossible to know the charging condition in advance, such that much inconvenience has been experienced. Charging of the lead storage battery has been done by supplying electric power from a generator, and a supply voltage has been controlled by a regulator to a constant value so as to avoid an overcharging. However, the voltage of the lead storage battery changes delicately depending on a discharge quantity, a temperature, a frequency of charge and discharge, and the history of the battery etc. For this reason, the overcharging of the battery can not be avoided by only a control of the voltage through means of the regulator i.e. only a control of voltage determined on the electric circuit side. This has been a major cause of the short service life of the lead storage battery for an automobile.
From the reasons as mentioned above, it has been strongly demanded to detect a charging condition or discharging condition of the lead storage battery.
As a method for detecting the charge and discharge conditions of lead storage battery, systems utilizing the following four methods for measuring a concentration of sulfuric acid forming an electrolyte have been known. However, any system has been expensive and not practical for use in the lead storage battery for automobile.
(1) Refractive index measuring method
A system of this method is composed of a light emitting diode, a light receiving diode and an optical path. This is a method for measuring a sulfuric acid concentration by making use of a property of sulfuric acid forming the electrolyte to change its refractive index according to its concentration. This method has already been put in practical use for a stationary lead storage battery. However, it has been impossible to minimize a size and reduce a cost because of necessity for executing a photoelectric conversion and preventing the optical path from being contaminated. For this reason, this method is not used for the lead storage battery for automobile.
(2) Specific gravity measuring method
This is a method for measuring a specific gravity of sulfuric acid by using a float. This is an inexpensive and easy method. In addition, this is a very useful and certain method in a manual operation such as maintenance. However, this method includes difficulties in respect of cost and structure in order to transmit measured data as electric signals to a data processor located at a center of automobile.
(3) Electrochemical method
This is a method in which an electrode system for sensor comprising components of metal, sulfuric acid and metal oxide is separately installed, and the concentration is measured utilizing such a property that an electromotive force depends on the sulfuric acid concentration. However, an appropriate electrode for this purpose can not be obtained yet. Only one put in practical use is an electrode system comprising components of lead, sulfuric acid and lead dioxide. However, this system is not appropriate for practical use because periodic reproduction of both electrodes is required.
(4) Electric conductivity method
This is a method for measuring an electric conductivity of sulfuric acid. However, since the electric conductivity of sulfuric acid concentration becomes a maximum in a condition where about a quarter of the sulfuric acid is discharged, the sulfuric acid concentration can not be determined from the electric conductivity unequivocally. In addition, it is required to process data in consideration of various factors such as stirred condition of electrolyte and temporary fluctuation due to external electric noise etc., so that this method is very complicated, expensive and lacks in reliability.
Further, the sulfuric acid forming the electrolyte of lead storage battery is strongly acidic, and an inside of battery is under a very strong oxidation-reduction atmosphere. For this reason, there are very many limitations in materials
This invention is made in consideration of the above-mentioned problems, and an object of this invention is to provide a sulfuric acid concentration sensor appropriate to a lead storage battery for an automobile which is small in its size, simple in its structure and cheap in its price.